Detecting Ovulation With a Basal Body Temperature Chart

Can BBT charting also detect early pregnancy?

A woman in bed looking at a thermometer, wondering if your BBT chart can tell her if she is pregnant
Michael H/Photodisc/Getty Images

You can use a basal body temperature (BBT) to conceive faster by determining your most fertile days. Detecting ovulation with basal body temperature (BBT) charting is relatively easy and inexpensive. Your gynecologist or reproductive endocrinologist may recommend charting to help detect when ovulation is happening or to get a better idea of your menstrual cycle patterns.

There are so many advantages of charting.

Charting can help you:

Here’s everything you could want to know about basal body temperature charting.

What Is BBT?

Your basal body temperature is your temperature when you’re at complete rest. Your basal body temperature changes based on a number of factors, including your hormones.

When you ovulate, the hormone progesterone causes your temp to rise. It remains higher throughout the two-week wait. Then, just before your period starts, the hormone progesterone drops. This means your basal body temperature will drop too—unless you’re pregnant, in which case your temperatures will remain higher because progesterone will stay high.

To know what your basal temp is, you must take your temperature in the morning before you get out of bed or move around. You can’t go to the bathroom quickly first. That will cause your temps to rise just slightly, enough to make your chart inaccurate.

It's essential that you take your temperature correctly.

Otherwise, your temperature will not be accurate, and you may not be able to detect ovulation. 

Choosing a BBT Chart

The first step to charting your basal body temperature is getting a chart to record your temperature.

You can find sample charts in some fertility books, such as Take Charge of Your Fertility (Harper Perennial, 1995)—a book considered by many to be the go-to resource for basal body temperature charting guidance.

Another option for charting is fertility awareness software, also known as fertility calendars. There are several fertility calendar options online, and several fertility apps for your phone. Many of them are free.

You could also make your own graph. If you make your own, you'll want to plot the temperature along the vertical, allowing one-tenth of a degree for each square. Along the horizontal, you'll have the days of your cycle.

Most women prefer using the computer because you can log a ton of information and reduce the chance of human error. Most ovulation software will automatically indicate when ovulation likely occurred. If you try plotting the temperature yourself, you might worry about making a mistake.

Once you have something to record your temperature on, it's time to start taking your basal body temperature.

How to Measure BBT

Now that you have a BBT chart picked out, you'll need to get a thermometer.

There are thermometers made especially for tracking your body basal temperature, but you might not need one. Ideally, you need one that is accurate to 1/10th (98.6) of a degree if you measure in Fahrenheit or 1/100th (37.00) of a degree in Celsius.

While some come with interesting features, the honest truth is that any good, regular thermometer will work.

Taking your basal body temperature isn't too hard. There are a few must-keep rules:

  • You need to take it at the same time (plus or minus no more than 30 minutes) every morning. For example, if you take it at 7:30 am, you don't want to take it earlier than 7 am or later than 8 am on other days.
     
  • You cannot get up, sit up, walk around, or go to the bathroom before taking your temperature. The minute after you wake up, you need to pop the thermometer in your mouth. (Which you hopefully placed within hand's reach the night before!)
     
  • You need to have had at least three to four straight hours of sleep before taking your temperature in the morning. If you stayed up all night, or you woke up and walked around at night repeatedly, it will throw off your results.
     
  • You should use the same thermometer throughout the cycle. (If you buy a new one, start using it on day one of the next cycle.)

If you want more details on taking your BBT temperature, read this article on how to take your basal body temperature.

When to Start Charting

Ideally, you should start charting on the first day of your period and continue to take your BBT temperature every morning throughout the entire cycle.

Every day, mark your waking basal body temperature, along with the time that you took your temperature.

After you have experience with charting, you may discover that you can skip the first few days of your period and start taking your temperature around day 5 or 7. Until you know when you tend to ovulate, though, it's best to take your temperature all the way through the cycle.

Identifying Ovulation

With basal body temperature charting, you're looking for an overall pattern, as opposed to a temperature spike here or there.

Your temperature may rise and fall as your cycle progresses, but you should notice a biphasic pattern after ovulation. This means that before ovulation, the temperatures are on average lower than they are after ovulation.

After you see at least three higher-than-average temperatures in a row, you can most likely say that ovulation occurred on the day before the first high temperature.

If you've been tracking your cervical mucus, then you can be even more sure ovulation occurred on the day before if you noticed fertile cervical mucus on the days leading up to the temperature rise.

If you're lucky, you may notice a sharp dip in temperature on the day of ovulation. Not every woman gets this nice heads up. If you do notice a consistent dip in temperature before the rise from month to month, you should be sure to have sexual intercourse on that day.

Getting Pregnant

The primary way to use a BBT chart to get pregnant is to look for patterns. Do you tend to ovulate on certain days of your cycle? Use this information to time intercourse better.

For example, if over a three-month period you note that ovulation occurred on days 11, 12 and 15, then on your next cycle, you probably want to time sex between days 6 through 16, with special attention toward days 11 through 15.

Remember, also, that you don't need to have sex on the day of ovulation to get pregnant.

If you have sex just a few times during those days before ovulation, that should be enough to get the sperm to the egg in time. Some couples try to have sex every other day the week before they expect ovulation. This is also a good plan.

Other Things to Track

To make charting most effective for you, you should track more than just your morning temperature. Here are some other things you may want to notice and indicate on your chart.

Here are some other things you may want to record on a BBT chart:

  • Days you have sex: This will help you and your doctor see if you're timing intercourse right. There are only five to seven days within each cycle when it's possible for sex to lead to pregnancy. The two to three days right before ovulation are best. You don't want to miss your window of opportunity.

    Another reason for charting when you have sex is to show how often you're having sexual intercourse. If male factor infertility is an issue, having sex every day may decrease your chances for pregnancy. On the other hand, having sex just once within the approaching days to ovulation may not be enough. Read more about how often to have sex.
     
  • Cervical mucus: If you track only one other thing besides your BBT, cervical mucus should be it. Here’s why. Your basal body temperature can only tell you that you ovulated after it happens. By the time you see three higher-than-average temperatures, your most fertile days have passed. However, your cervical mucus can tell you when you are about to ovulate. You can use that information to know when to have sex.
  • Cervical position: Besides tracking your cervical mucus, you can also track your cervical position to help predict ovulation. Your cervix will become higher, softer, and more open as ovulation approaches. After ovulation, the cervix becomes firmer, lower, and closed (or partially closed).
      
  • Illness, stress, or difficulty sleeping: Even a relatively benign cold can mess with your BBT charting. If sinus congestion forces you to sleep with your mouth open, for example, your temperature may be thrown off. Also, poor sleeping habits can skew the results.
     
  • Ovulation predictor kit results: If you're also using an ovulation predictor kit or any other ovulation prediction technology, you should mark down these results on your chart.

What If You’re Not Ovulating?

One of the advantages of charting is you can see whether you are ovulating. Signs on your chart that may indicate that you’re not ovulating include…

If you're not ovulating, you can't get pregnant. If you are ovulating irregularly, it may indicate a possible infertility risk

Lack of ovulation is called anovulation and is a common cause of female infertility.

Most women with anovulation can take fertility drugs, which will trigger ovulation and hopefully help them get pregnant. Clomid is the most well-known fertility drug used to treat anovulation.

Pregnant BBT Chart

Can your BBT chart tell you whether you’re pregnant or not? Yes and no. Many women read into every little temperature fluctuation. It's part of the two-week wait obsession and the never-ending search for early pregnancy signs.

There are four ways a BBT chart can indicate a pregnancy the possibility of pregnancy.

Whether you had sex on your most fertile days: Your basal body temperature can’t predict ovulation. You can only know if and when you ovulated a few days after it happened in a BBT chart. This means you may not know if you had sex on the “right days” until after ovulation occurs.

You can look back on your chart and determine this. You are most likely to conceive if you had sex on the two days preceding ovulation.

If you have an implantation dip on your BBT chart: An implantation dip is a one-day drop in temperature about a week after ovulation. The majority of the time, an implantation dip is nothing more than a mid-cycle dip in temperature and does not indicate pregnancy. It’s debatable whether or not this is a possible sign of early pregnancy.

If you have a triphasic pattern on your BBT chart: A triphasic temperature pattern is a second temperature increase occurring about one week after ovulation. Seeing a triphasic pattern on your BBT chart is slightly more likely to indicate a potential pregnancy, but it is also no guarantee. A triphasic pattern indicates that progesterone rose a little bit more, causing your temperatures to also rise slightly more. This may occur because you’re pregnant. That said, it also could happen when you’re not.

If your luteal phase is longer than normal: The most reliable way to detect pregnancy on a BBT chart takes patience. The old-fashioned method: By waiting to see if your luteal phase—the time between ovulation and your expected period—is longer than usual.

For most women, their luteal phase does not vary by more than a day or two from month to month, even if the length of their menstrual cycle does vary. For example, a woman’s cycle may vary between being 30 and 35 days, but her luteal phase may consistently be 12 or 13 days long.

If you see that your luteal phase has gone at least one day past the usual length, you might be pregnant. If it goes two days past the longest luteal phase you’ve ever had, the likelihood of being pregnant is even higher. This is a good time to take a pregnancy test.

If you reach 18 days past ovulation and you still don’t have your period, chances are very good that you are pregnant. Not many women can wait that long without taking a pregnancy test. Still, it is the strongest early sign of pregnancy detectable with a BBT chart.

A Word From Verywell

Basal body temperature charting is a great way to track your cycles and ovulation patterns. It can also help your doctor detect possible ovulatory infertility. If you are concerned you aren’t ovulating, bring your BBT chart to your gynecologist.

When it comes to detecting pregnancy, BBT charts can only offer small hints. You can't confirm pregnancy with a fertility calendar. We know how tempting it can be to look for early signs of pregnancy, and how stressful it can be waiting to take a pregnancy test. However, since there are no reliable ways to detect conception without "peeing on a stick," the best way to use your time and energy during the two-week-wait is to focus on self-care and to distract yourself with your life beyond trying to conceive.  

Sources:

Colombo B, Masarotto G. “Daily fecundability: first results from a new data base." Demogr Res. 2000 Sep 6;3:[39] p.

Fertility Awareness: Natural Family Planning. American Pregnancy Association.

Mayo Clinic. Basal body temperature for natural family planning.

Patient Fact Sheet: Ovulation Detection. American Society of Reproductive Medicine.

Triphasic Pattern and Pregnancy: a Statistical AnalysisFertilityFriend.com